The invention relates to a joined structure of a ceramic heater and an electrode terminal, and a joining method therefor.
With higher speed and higher pin counts of semiconductor devices, semiconductor implementation technology providing narrower pitch and higher accuracy has been becoming more and more important.
In particular, the flip chip bonding (FCB) technique shown in FIG. 7 (a) has been used for the COG in liquid crystal panel package and MCM implementation in computers and portable telephones. In this technique, a Si chip 50 of a semiconductor device and an electrode 62 on a board 60 are directly bonded at a bonding terminal (bump) 52 of solder and Au solder by means of a thermocompression technique.
This flip chip bonding (FCB) technique provides more compact, smaller and faster devices than the wire bonding (WC) and tape carrier (TC) techniques in which conventional lead wires 54 (FIG. 7b), 56 (FIG. 7c) are used for bonding. Therefore, it is expected that the flip chip bonding technique will be mainstream instead of the wire bonding technique (WC) in the future.
FIG. 6 illustrates an example of apparatuses used for flip chip bonding.
When flip chip bonding is performed, a tool head 20 of the size corresponding to the Si chip 50 is selected and fixed to a heater 1 by vacuum absorption. Next, the Si chip 50 is attached to the tool head 20, also by vacuum absorption.
Therein, the heater 1 is secured to a jacket 30 for forced-cooling of the Si chip 50 by screws 11.
Then, a board 60, on which an electrode 62 to be bonded to the Si chip 50 is disposed, is secured to a board stage 40 by vacuum absorption.
Then, the tool head 20 is lowered in the direction perpendicular to the board stage 40, and positioned until the bonding terminal 52 on the Si chip 50 is brought into contact with the electrode 62 on the board 60. Thereafter, the predetermined load (about maximum 50 kgf) is applied to the tool head 20 and at the same time the Si chip 50 is rapidly heated to a predetermined temperature (from 50.degree. C. to approximately 450-500.degree. C. about 5 seconds) which is then kept for a given time (about 3 to 5 sec) . Thus, the bonding terminal (bump) 52 on the Si chip 50 and the electrode 62 on the board 60 are thermo-compressed to each other.
And then, the power of the heater 1 is switched off without delay, and forced cooling (water-cooled or air-cooled) of the jacket 30 rapidly lowers the temperature of the Si chip 50 placed on the tool head 20 (from 450-500.degree. C. to100.degree. C. about 20 sec). Thereby, the bonding terminal (bump) 52 on the Si chip 50 and the electrode 62 on the board 60 are bonded together. Thus, the flip chip bonding (FCB) is completed.
In this case, in order to prevent the bonding terminal (bump) 52 on the Si chip 50 from being spread and at the same time to reduce thermal damage to the Si chip 50, rapid temperature rise and drop (temperature rise: less than or equal to 5 sec., temperature drop: less than or equal to 20 sec.) are essential for ensuring flip chip bonding (FCB).
For meeting the conditions described above, recent bonding heaters 1 mainly used are made of ceramics such as aluminum nitride, silicon carbide, and silicon nitride which are excellent in the characteristics of thermal conductivity, heating uniformity, heat dissipation ability, and thermal shock resistance.
So, it is required in the method for joining an electrode terminal to the heater 1 described above that the joint portion is vacuum-tight such that a heating element embedded in the heater may be protected from high temperature air, in addition to high electric conductivity and having enough joining strength.
To this end, when a joining metal of the electrode terminal is joined to the heater, it is necessary to join the heating element and the ceramic base material surrounding it at the same time. Therefore, conventionally, active metal solder has been used for joining them.
However, as shown in FIG. 3, if the joining metal of a ceramic base material 2 and an electrode terminal 80 are joined directly to each other by a metal solder 90, active metals in the metal solder 90 react with the joining metal of the electrode terminal 80. So, this reaction causes a deficiency of active metals which can react with ceramic base material 2, and so results in poor wettability between the ceramic base materia l2 and the metal solder 90. Consequently, there have been problems of a remarkable reduction in the joining strength between them and the short life of the heater.
In particular, these problems are outstanding in electrode terminals made of metals such as kovar and SUS, which include as main ingredients one or more kinds of the groups consisting of Fe, Ni, Co.
The present invention has been performed, considering such problems of the conventional joining technology. An object of the invention is to provide a joined structure and a joining method therefor which allow high reliability of the joining strength between a heater and an electrode terminal and also the longer life of the heater.